The invention relates to a method of treating gaseous waste containing germanium and coming from a unit for manufacturing an optical fiber preform.
In conventional manner, a unit for manufacturing an optical fiber preform causes gases of the silicon chloride SiCl.sub.4 and germanium tetrachloride GeCl.sub.4 type to react with oxygen at high temperature. The resulting silicon oxide SiO.sub.2 is deposited in successive layers to form a cylindrical preform. Germanium oxide is deposited in the central layers of the preform so as to form a core that has a refractive index different from that of the silica.
Only a small fraction of the germanium tetrachloride reacts with oxygen. The germanium tetrachloride that has not reacted, and the germanium oxide that is not deposited, are discharged by the manufacturing unit together with hydrogen chloride HCl. These gases cannot be discharged as such into the environment. That is why it is necessary to treat them.
Furthermore, it is also desirable to recover the germanium, not only to preserve environment, but also to recycle the germanium for subsequent reuse in the manufacture of optical fiber preforms so as to reduce manufacturing costs.
A method known from document U.S. Pat. No. 4,385,915 serves to treat such gaseous waste and to recover the germanium. It comprises the following steps in particular:
forming a liquid effluent from the gaseous waste, e.g. by causing the gaseous waste to react with caustic soda; the liquid effluent then contains not only germanium, but also hypochlorite; PA1 reducing the hypochlorite, e.g. by adding oxygenated water; and PA1 precipitating the germanium in the liquid effluent treated in this way by adding magnesium sulfate. PA1 forming a liquid effluent from said gaseous waste; and PA1 precipitating the germanium in said liquid effluent; PA1 the method being characterized in that said precipitation is performed by adding magnesia MgO to said liquid effluent.
The precipitation of the germanium contained in the effluent by means of the magnesium sulfate releases sulfates in the supernatant phase, which sulfates are compounds that are harmful to the environment.